1. Field of the Invention
The present invention relates to a developing device for a copier, printer, facsimile apparatus or similar image forming apparatus and an image forming apparatus using the same. More particularly, the present invention relates to a developing device of the type causing a developer to form a magnet brush on a developer carrier in a developing zone where the developer carrier faces an image carrier to thereby develop a latent image formed on the image carrier, and an image forming apparatus using the same.
2. Description of the Background Art
It is a common practice with an electrophotographic, electrostatic or similar image forming apparatus to form a latent image on a drum, belt or similar image carrier in accordance with image data and develop it with a developing device for thereby producing a corresponding toner image. Today, a two-ingredient type developer made up of toner and carrier is predominant over a single-ingredient type developer, i.e., toner because it is desirable in image transferability, halftone reproducibility, and stability against temperature and humidity.
In a developing device of the type using a two-ingredient type developer, the developer is caused to rise on a developer carrier in the form of a magnet brush and conveyed to a developing zone where the developer carrier faces an image carrier. In the developing zone, the magnet brush rubs the surface of the image carrier with the result that the toner is fed from the magnet brush to a latent image formed on the image carrier for thereby developing the latent image.
In the developing device of the type described, the developer carrier is usually made up of a cylindrical sleeve and a magnet roller accommodated in the sleeve and provided with a plurality of magnetic poles. The magnet roller forms a magnetic field for causing the developer to rise on the sleeve surface in the form of a magnet brush. The sleeve moves relative to the magnet roller for thereby conveying the developer to the developing zone. In the developing zone, the developer forms brush chains along magnetic lines of force issuing from the magnetic pole for development, forming a magnet brush. The magnet brush contacts the surface of the image carrier while deforming in accordance with the movement of the sleeve surface, thereby feeding the toner to the latent image.
As the distance between the image carrier and the sleeve in the developing zone decreases, image density increases while a so-called edge effect decreases, as known in the art. In this sense, the above distance should be as small as possible. However, when the distance is reduced, it is likely that the trailing edge of a black or halftone solid image is lost or that the reproducibility of thin lines is lowered, degrading image quality.
In the developing zone, the surface of the sleeve moves in the same direction as, but at a higher linear velocity than, the surface of the image carrier. Therefore, the magnet brush moves relative to the latent image of the image carrier in such a manner as to rub the latent image while outrunning it. Paying attention to a portion of the latent image corresponding to the tailing edge of an image, brush chains rubbing the above portion one after another have a toner feeding ability that sequentially decreases, as will be described more specifically hereinafter.
Part of the magnet brush entered the developing zone and rubbing the trailing edge portion of the latent image is the part that has faced the non-image portion of the image carrier positioned at the upstream side in the direction of movement of the image carrier. On the tips of brush chains forming the above part of the magnet brush, toner grains deposited on carrier grains have been shifted toward the sleeve due to the electrostatic force of the non-image portion. This phenomenon is generally referred to as toner drift. Toner drift becomes more noticeable as a period of time over which the brush chains face the non-image portion increases. As a result, the brush chains rubbing the trailing edge portion of the latent image at the downstream portion of the developing zone have faced the non-image portion over a longer period of time than the brush chains rubbing it at the upstream side of the developing zone. It follows that toner drift is more conspicuous on the former brush chains than on the latter brush chains and reduces the number of toner grains present on the individual carrier grain, thereby reducing the toner feeding ability.
Subsequently, when the trailing edge portion of the latent image moves out of the developing zone, the brush chains rubbing it have hardly any toner grains on their carrier grains. When toner drift on the brush chains goes so far, the carrier grains of the brush chains electrostatically attract toner grains deposited on the trailing edge portion of the latent image. Consequently, despite that the toner grains have been fed from the brush chains to the trailing edge portion of the latent image, the toner grains are returned to the other brush chains having hardly any toner on the carrier grains before they leave the developing zone. This is presumably the cause of the omission of the trailing edge and the degradation of thin line reproducibility.
To reduce the omission of the trailing edge of an image and the degradation of thin line reproducibility, Japanese Patent Laid-Open Publication Nos. 2000-305360, 2000-347506 and 2001-5296, for example, each propose a particular attenuation ratio of a flux density in the normal direction in the developing region, a particular angular distance between a main magnetic pole for forming a magnet brush and a magnetic pole adjoining it, and a particular half-value center angle of the main pole. More specifically, a single main magnetic pole (N pole) and two auxiliary magnetic poles (S poles) respectively positioned upstream and downstream of the main pole in the direction of movement of the sleeve surface constitute the magnetic pole for development.
Japanese Patent Laid-Open Publication No. 2001-27849 proposes a particular nip for development and particular density of a magnet brush. Also, Japanese Patent Laid-Open Publication No. 2001-134100 proposes a particular half-value angular width or half-value center angle of a main magnetic pole. With such particular configurations, it is possible to enhance developing efficiency, reduce the omission of the trailing edge of an image, and improve thin line reproducibility.
In accordance with the prior art technologies stated above, to enhance developing efficiency, reduce the omission of the trailing edge of an image and improve thin line reproducibility, the ratio of the linear velocity of the sleeve to that of the image carrier, as measured in the developing zone, is increased to allow a sufficient amount of toner to be fed to a latent image.
While the linear velocity ratio mentioned above may be increased by lowering the linear velocity of the image carrier or raising the linear velocity of the sleeve, the latter scheme is usually used because the former scheme lowers image forming speed. However, when the linear velocity of the sleeve is raised, a centrifugal force acting on the developer deposited on the sleeve is intensified. As a result, carrier grains forming the magnetic brush are apt to part from the magnet brush due to, e.g., a shock to occur when the magnet brush contacts the image carrier, flying out of the developing device. This phenomenon will hereinafter be referred to as carrier scattering. The carrier grains flown out of the developing device deposit on the image carrier and various parts and devices arranged therearound. The carrier grains deposited on the image carrier disturb an image or cause the dots of an image to be partly lost, thereby lowering image quality. In addition, the carrier grains deposited on parts and devices around the developing device are likely to damage them.
Today, there are extensively used an image forming apparatus with relatively high image forming speed in which the linear velocity of the image carrier is between 100 mm/sec and 300 mm/sec (medium speed) and an image forming apparatus with high image forming speed in which the linear velocity is between 300 mm/sec and 600 mm/sec (high speed). In such a medium-speed or a high-speed image forming apparatus, the linear velocity of the sleeve and therefore centrifugal force to act on the developer deposited on the sleeve are further increased, so that the problems discussed above are more likely to occur.
By a series of researches and experiments, we found that in the conventional developing devices an electrostatic force exerted by the image carrier caused the carrier grains positioned on the tips of the brush chains in the developing zone to deposit on the image carrier. More specifically, it has been customary to cause the magnet brush to rub, or move relative to, the surface of the image carrier for thereby feeding more toner to a latent image than when the magnet brush moves at the same speed as the surface of the image carrier. In this condition, in the developing zone, part of the magnet brush not contacting the surface of the image carrier, i.e., adjoining the sleeve moves relative to the surface of the image carrier. However, the other part of the magnet brush contacting the surface of the image carrier, in many cases, adhere to the surface of the image carrier, but does not rub it. Therefore, the effect achievable with the conventional developing device is limited. This is also true with a developing device in which the magnet brush is short and is dense in its portion contacting the image carrier. The effect achievable with this kind of developing device is also limited even when the linear velocity ratio of the sleeve to the image carrier is increased.
As for carrier scattering, experiments showed that not only the centrifugal force but also the following two factors should be taken into account. First, in the developing zone, the carrier grains on the tips of the brush chains are subject to the composite force of the centrifugal force and electrostatic force and tend to part from the magnet brush. Second, the above carrier grains are subject to the composite force of the centrifugal force and gravity and also tend to part from the magnet brush. These factors will be described more specifically later.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-open Publication Nos. 2000-47476, 2000-305355, 2001-27829, 2001-290305, 2002-268386 and 2002-287503.